1. Field of the Invention
This invention relates to the field of pulsed magnetic electronic article surveillance (EAS) systems in general, and in particular, to pulsed magnetic EAS systems having one antenna for both transmitting and receiving with independent phasing.
2. Description of Related Art
In conventional EAS systems, transmit and receive functions are provided by separate antenna coils. The need for separate antenna coils is problematic. The manufacturing cost is higher due to the additional material cost, assembly and test time, as well as the additional documentation and quality checking that is required.
The transmit and receive fields sometimes do not coincide. As an example, when a tag or marker is located in an orientation or location wherein it is sufficiently stimulated by the transmit antenna field, the tag or marker may not be simultaneously in an orientation or location wherein the tag or marker can be optimally interrogated by the receive antenna.
In transceiver antenna assemblies, especially those wherein separate transmit and receive coils operate in close proximity, the magnetic field produced by the transmit coil induces a current in the nearby receiver coil. This current in the receiver coil can be significant due to the low impedance of protective shunting devices commonly employed to protect the receiver input from damaging potentials due to transformer action between the two coupled coils. This induced current flowing in the receiver coil produces a magnetic field of its own, which is in opposition to the field produced by the transmit antenna. Due to the interaction of the opposing fields produced by the transmit antenna and the receive antenna, the net magnetic field in the interrogation zone is diminished, resulting in a loss of efficiency and requiring higher transmitter power to establish the desired field strength.
In conventional EAS systems employing multiple transmitter coils, the transmitter field phasing relationship is fixed or selectable during the installation process. Accordingly, it is possible for a tag or marker to pass through the interrogation zone in an orientation or location such that it is not sufficiently stimulated to be detected by the receive antennas. Transmitter antennas used in magnetic EAS systems must typically be configured as resonant circuits in order to develop enough current to provide sufficient field strength for proper operation. When multiple transmit coils are operated in close proximity, their fields interact, resulting in impedance changes in each coil. Once multiple transmit antennas are tuned to resonance for a particular phase relationship, if this phase relationship is changed significantly, each of the coils will no longer be at its resonant peak and each coil's current will be reduced by an amount depending on the magnitude of the phase difference, the individual coil's quality factor (Q) and the coefficient of coupling between the respective coils. In a multi-coil transmitter antenna, the transmitter coils must be resonated or tuned each time the field phasing changes substantially.
Most existing EAS systems utilize separate antenna coils for the transmit and receive functions of the system. The transmitter field phase relationship between coils is either fixed or, if electronically controlled, held constant during system operation.
The Ultra*Max product line available from Sensormatic Electronics Corporation does include a DoubleChecker device for reminding clerks to remove or deactivate magnetic labels or markers from protected items during the checkout process. The terms Ultra*Max and DoubleChecker are trademarks of Sensormatic Electronics Corporation. The antenna used in the DoubleChecker performs a dual function, but is conceptually and electrically different from the single transmit and receive antenna implementations explained above, and is unsuitable for use in a full system due to prohibitive reactive voltages.
A coplanar single coil dual function transmit and receive antenna for a proximate surveillance system is shown in U.S. Pat. No. 4,963,880 and comprises: a single coil having a perimeter and enclosing a unique region; means for driving the coil during the transmit intervals to generate and transmit time varying magnetic field components within the proximate region, the coil being driven on, producing components vertical and horizontal to the plane of the coil, which components are the result of fringing effects within the area approximated by the coil perimeter; circuit means connected to the coil for forming with the coil, during the transmit intervals, a series resonant tuned circuit and, during the receive intervals, an untuned circuit; and, tristate output switched-mode operating means connected to the circuit means for providing an intrinsic automatic changeover of the circuit means between the tuned and untuned circuits.
Some pulsed magnetic EAS systems, for example those available from Sensormatic Corporation, synchronize their operation by sensing the local power line positive zero crossings, as shown in FIG. 5. Each line cycle is divided up into six alternating time windows: three windows for transmission and three windows for reception. The first transmit-receive window sequential pair occurs at 0.degree. with respect to the zero crossing, the second first transmit-receive window sequential pair occurs at 120.degree. with respect to the zero crossing and the third first transmit-receive window sequential pair occurs at 240.degree. with respect to the zero crossing.